No. EP-A-0 032 362 describes a display device, whose electrooptical display material is a chiral smectic C phase liquid crystal. This disply device, shown diagrammatically in longitudinal section in FIG. 1, has a first linear polarizer 2 and a second linear polarizer 4, which are crossed and between which is inserted a tight display cell 6. A light source 8 below polarizer 4 makes it possible to illuminate cell 6. This display cell operating in transmission is formed from two generally glass transparent insulating walls 10, 12. These parallel walls are joined by their edges using a weld 14 as a sealing joint.
Walls 10 and 12 are respectively covered with an electrode 16 and a counterelectrode 18 with an appropriate shape for display purposes and made from a transparent conductive material. The electrode and counterelectrode are in each case more particularly formed from parallel conductive strips, the strips of the electrode and the strips of the counterelectrode crossing one another. Said electrode and counterelectrode make it possible to apply to the terminals of a liquid crystal film 20 with a chiral smectic C phase and contained in cell 6, a continuous electric field E, whose direction or polarity can be changed. For this purpose electrode 16 and counterelectrode 18 are connected via an inverter 22 to a continuous power supply 24.
FIG. 2 shows on a molecular scale, the structure of a liquid crystal film with a smectic C phase, when the latter is contained in a display cell 6. With a view to simplifying FIG. 2, only the cell walls 10 and 12 are shown. The lower wall 12 e.g. constitutes a reference plane containing the two axes X and Y of an orthogonal reference system XYZ.
The smectic C liquid crystal film is formed from elongated molecules 26 having a longitudinal axis 28 and arranged in the form of layers 30. Each of these molecules has a permanent dipole moment p perpendicular to the longitudinal axis 28 thereof.
In the ideal case shown in FIG. 2, the smectic layers 30 are all parallel to one another and oriented perpendicularly to cell walls 10 and 12.
On applying an electric field E to such a liquid crystal, a high coupling is obtained between the molecular orientation (longitudinal axis 28 of the molecules) and said electric field E due to the presence of the permanent dipole. This coupling is of the polar type, because the electric dipole is preferably oriented in a direction parallel to the electric field. The polarity change of the electric field consequently makes it possible to change the orientation of the electric dipole and therefore the orientation of molecules 26.
FIG. 2 shows in continuous line form the molecules 26 of the liquid crystal according to a first orientation A.sub.1 (state 1) forming an angle -.theta. with respect to direction X, the dipole moment p being oriented perpendicular to walls 10 and 12 of the cell and in direction 10-12 of electric field E. The polarity change of the electric field makes it possible to switch dipole moments p into direction 12-10 leading to a pivoting of the molecules about axis Z by an angle of 2.theta.. The second orientation A.sub.2 of the molecules (state 2) is symbolized in mixed line form. It forms an angle +.theta. with respect to direction X.
The molecules pass from the first to the second orientations and vice versa, whilst describing a cone angle at the apex of 2.theta. characteristic of the material (typically .theta.-=22.5.degree.).
FIG. 2 also shows the polarization directions P and P', respectively of rectilinear polarizers 2 and 4. When these two polarizers are crossed and when in state 1 the liquid crystal molecules 26 are parallel to polarization direction P' of polarizer 4, the optical state 1 of the liquid crystal corresponds to the absorbtion of the light from supply 8 and optical state 2 to the transmission of said same light.
The appropriately oriented chiral smectic C phase liquid crystals (FIG. 2) can thus be used as a display material. Apart from their bistability, they can have interesting properties, such as a rapid switching or response time of approximately 1 microsecond for small voltages applied to the electrodes (a few volts) and a wide electrooptical response.
Unfortunately, the presently known display devices using as the display material a chiral smectic (H or C) phase liquid crystal have a certain number of constructional problems mainly linked with the presence of the permanent dipole moment of the molecules.
Thus, in the absence of an electric field, the dipole moments of the liquid crystal molecules interact with the dipoles of cell walls 10 and 12 and possibly with those of the electrodes. In the case of two identical walls with a limited surface tension, this can lead to the helical arrangement of the molecules in each smectic layer 30, as shown in FIG. 3, the first molecule 26' of each smectic layer 30 oriented in accordance with A.sub.2 and in contact with cell wall 10 and the last molecule 26.sub.n of said same layers oriented in accordance with A.sub.1 and in contact with cell wall 12 being displaced by an angle 2.theta..
This orientation characterized by a twisting of the molecules in each smectic layer 30 is stable in the absence of the electric field. This liquid crystal has no memory effect, because a given orientation of the molecules (state 1 or 2), in the presence of an electric field, cannot be maintained on eliminating said field. This helical molecular configuration does not make it possible to produce display devices with an internal memory. Moreover, a homogeneous orientation of all the molecules of the liquid crystal is difficult to obtain for large display cells.